Most of the baryons (e.g. H, He, C, N, O) in the Universe, at all epochs and especially at high redshift, are not contained within stars or galaxies but are in the intergalactic medium(IGM). The primary observational tool for deriving the physical properties (temperature, kinematics, ionization state, metal content) of these baryons is via absorption lines imprinted on the spectra of background quasars.

High-resolution quasar spectra, together with numerical simulations, have demonstrated that the IGM is arranged in a "cosmic web" of filaments and voids, with galaxies forming in the densest regions. Research in this area at the institute uses quasar spectra to try and understand how the IGM evolves with time, and how galaxies and the IGM interact. One of the areas that we focus on is the  study the processes by which hydrogen inthe IGM is reionized by the first galaxies and helium is reionized by quasars. Recent projects have included determinin the total mass of neutral hydrogen; charting the
thermal history of the IGM, searching for the chemical signatures of the first galaxies, and measuring the total output of ionizing photons